1. Field of the Invention
The present invention relates to a method for fabricating a circuit device, and in particular, a method for fabricating a circuit device, which is able to achieve thinning of the circuit device using two sheets of conductive layer.
2. Description of the Prior Arts
Recently, IC packages have been actively employed in portable devices, and small-sized and high density assembly devices. Conventional IC packages and assembly concepts tend to greatly change. For example, this is described in, for example, Japanese Laid-Open Patent Publication No. 2000-133678. This pertains to a technology regarding a semiconductor apparatus in which a polyimide resin sheet being a flexible sheet is employed as one example of insulation resin sheets.
FIG. 9 through FIGS. 11A, 11B and 11C show a case where a flexible sheet 50 is employed as an interposer substrate. Also, the views illustrated upside of the respective drawings are plan views, and the views illustrated downside thereof are longitudinally sectional views taken along the lines Axe2x80x94A of the respective drawings.
First, copper foil patterns 51 are prepared to be adhered to each other via an adhesive resin on the flexible sheet 50 illustrated in FIG. 9. These copper foil patterns 51 have different patterns, depending upon cases where a semiconductor element to be assembled is a transistor or an IC. Generally speaking, a bonding pad 51A and an island 51B are formed. Also, an opening 52 is provided to take out an electrode from the rear side of the flexible sheet 50, from which the above-described copper foil pattern 51 is exposed.
Subsequently, the flexible sheet 50 is transferred onto a die bonder, and as shown in FIG. 10, a semiconductor element 53 is assembled or mounted. After that, the flexible sheet 50 is transferred onto a wire bonder, wherein the bonding pads 51A are electrically connected to the pads of the semiconductor elements 53 by thin metal wires 54.
Finally, as shown in FIG. 11A, sealing resin 55 is provided on the surface of the flexible sheet 50, and the surface thereof is completely sealed with the sealing resin 55. Herein, the bonding pads 51A, island 51B, semiconductor elements 53 and thin metal wires 54 are transfer-molded so as to be completely overcoated.
After that, as shown in FIG. 11B, connecting means 56 such as solder and a soldering ball is provided, wherein spherical solder 56 deposited to the bonding pad 51A is formed via the opening 52 by passing through a solder reflow furnace. Further, since semiconductor elements 53 are formed in the form of a matrix on the flexible sheet 50, these are diced to be separated from each other as shown in FIG. 11.
In addition, the sectional view of FIG. 11C shows electrodes 51A and 51D on both sides of the flexible sheet 50 as the electrodes. The flexible sheet 50 is generally supplied from a maker after both sides thereof are patterned.
In prior art methods for fabricating circuit devices, a flexible sheet 50 is transferred in the above-described fabrication apparatus, for example, a die bonder, a wire bonder, a transfer molding apparatus, a reflow furnace, etc., and is mounted on a portion called a xe2x80x9cstagexe2x80x9d or a xe2x80x9ctablexe2x80x9d.
The thickness of insulation resin, which becomes the base of a flexible sheet 50, is made thin at approx. 50 xcexcm, and where the thickness of a copper foil pattern 51 formed on the surface thereof is thin at 9 through 35 xcexcm, there is a shortcoming by which the insulation resin is warped as shown in FIG. 12 to cause its transfer performance to be worsened, and mountability thereof on the above-described stage or table is also worsened. It is considered that this is because the insulation resin itself is thin to be warped, and warping occurs due to a difference in the thermal expansion coefficient between the copper foil pattern 51 and the insulation resin. In particular, there is another problem in that, if a hard insulation material not using any core material of glass cloth fibers is warped as shown in FIG. 12, the insulation material is easily collapsed by compression from above.
Since the portion of the opening 52 is compressed from above when being molded, a force by which the periphery of the bonding pad 51A is warped upward is brought about, the adhesion of the bonding pad 51A is worsened.
Also, the resin material that constitutes a flexible sheet 50 has less flexibility, or if a filler to increase the thermal conductivity is blended, the flexible sheet 50 is made hard. In such a case, where bonding is carried out by a wire bonder, there may be a case where the bonded portion is cracked. Also, when performing transfer molding, there is a case where the portion with which a metal die is brought into contact is cracked. This remarkably occurs if any warping shown in FIG. 12 is provided.
Although the flexible sheet 50 described above is such a type that no electrode is formed on the rear side thereof, there are cases where an electrode 51D is formed on the rear side of the flexible sheet 50 as shown in FIG. 11C. At this time, since the electrode 51D is brought into contact with the above-described fabrication apparatus or is brought into contact with the transfer plane of transfer means between the fabrication apparatuses, another problem occurs in that damage and scratches arise on the rear side of the electrode 51D, wherein the electrode is established with such damage and scratches retained, the electrode 51 itself may be cracked due to application of heat later on.
Also, if an electrode 51D is provided on the rear side of the flexible sheet 50, a problem occurs in that, when carrying out transfer molding, no facial contact with the stage can be secured. In this case, if the flexible sheet 50 is composed of a hard material as described above, the electrode 51D becomes a fulcrum and the periphery of the electrode 51D is compressed downward, wherein the flexible sheet 50 is cracked.
A method for fabricating a circuit device according to the invention is comprised of the steps of: preparing an insulation resin sheet having a first conductive layer and a second conductive layer adhered together by insulation resin; forming a conductive path layer by etching the above-described first conductive layer to an appointed pattern; adhering and fixing semiconductor elements on the above-described conductive path layer with the same electrically insulated therefrom; overcoating the above-described conductive path layer and the above-described semiconductor elements with a sealing resin layer; removing the above-described second conductive layer by etching the same; and providing through holes in the above-described insulation resin that covers the rear side of the above-described conductive path layer, and forming external electrodes on the above-described conductive path layer, whereby the above-described problems can be solved.
Since the flexible sheet is formed to be thick by the first conductive layer and the second conductive layer, flatness of a sheet-like circuit device can be maintained even if the insulation resin is thin.
Also, mechanical strength is retained by the second conductive layer until the step of overcoating the first conductive path layer and semiconductor elements with a sealing resin layer is finished, and the sealing resin layer retains the mechanical strength thereafter. Therefore, the second conductive layer can be easily removed. As a result, the insulation resin does not require any mechanical strength, wherein the insulation resin can be made sufficiently thin to retain only an electrical insulation property.
Further, since the lower die mold and planes of a transfer molding apparatus are brought into contact with the entirety of the second conductive layer, no local compression is brought about, and it is possible to prevent the insulation resin from being cracked.
The method according to the invention has the following advantages.
First, warping of an insulation resin sheet can be prevented by the second conductive layer until a substrate is molded by a sealing resin layer, and transfer performance thereof can be improved.
Second, since the second conductive layer can retain mechanical support of the insulation resin sheet until the sealing resin layer is formed, and the sealing resin layer can subsequently retain mechanical support of the insulation resin sheet after the second conductive path layer is formed, the mechanical strength of the insulation resin is disregarded, wherein a remarkably thin assembly method can be achieved.
Third, since both sides of the insulation resin are covered with the first and second conductive layer in cases where the insulation resin itself is hard or becomes hard by blending of a filler, the flatness of the insulation resin sheet itself can be improved, wherein it is possible to prevent cracks from occurring.
Fourth, since the rear side of the insulation resin sheet has the second conductive layer thickly formed, the insulation resin sheet can be used as a supporting substrate for die bonding of chips and for sealing a wire bonder and semiconductor elements. Further, even where the insulation resin material is soft, propagation of energy in wire bonding can be improved, wherein the wire bondability can be accordingly improved.
Fifth, since the conductive path layer can be finely patterned and can be freely routed below the semiconductor elements, path density which is equivalent to multi-layer connection can be realized in a single layer although so-called multi-layer connection is not enabled. In addition, overcoating on the surface where external electrodes are formed can be abolished, and no plating process is required, wherein a remarkably cheap and simplified fabrication process can be achieved.